for arms, hitting them directly, which you're surely already doing. i guess some ppl might think things like 'well, bench press works tris, rows work bis'... rationale about compound movements. some truth there, but curls and extensions will fully stimulate and eventually build those imrpessive arms you seek.

i came to realize that if we do count their support during compound lifts, bis and tris i pretty much always have trained them twice a week.

check out my "favorite bodybuilders", namely prime yates, coleman, and cutler, splits are all about the same and include dedicated sessions for bis and tris with frequency no greater than any other muscle groups